L 28 Electricity and Magnetism [5]

1. L 28 Electricity and Magnetism [5] • magnetism • magnetic forces • applications Why are magnets magnets?

2. Magnetism • two sources of magnetism • permanent magnets • electromagnets • the earth’s magnetic field • how does a compass work • the north pole is really a south pole! • Van Allen radiation belts

3. Permanent magnetism • certain minerals (loadstone) are naturally magnetic • a piece of loadstone will attract bits of iron • a magnet produces a magnetic field in the space around it, just like the Sun produces a gravitational field that holds the planets in their orbits • the magnetic field can be visualized with iron filings

4. N S S S N N Permanent magnets • Are made from alloys of some of the rare earth elements like neodymium and samarium • Always have a north and a south pole • like poles repel and unlike poles attract • if you break a magnet in half you get 2 magnets

5. If you pass current through a loop of wire you get a magnet  Oersted’s discovery  currents make magnetic fields (Ampere’s Law) BarMagnet Electromagnet

6. + Duracell Homemade magnets COIL Iron nail You can think of the nail as a collection of little magnets that are randomly aligned. The magnetic field of the coil aligns these little magnets giving a larger field than that of the coil alone. We say that the nail becomes “magnetized”, but the effect is not permanent.

7. N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S N S UNMAGNETIZED atomic magnets PARTIALLY MAGNETIZED NORTH SOUTH MAGNETIZED

8. Magnetic materials • some materials are naturally magnetic or can be magnetized and retain their magnetism  ferromagnetic materials • other materials (iron) can be magnetized temporarily by placing them near magnets • some materials have essentially no magnetic properties  copper, aluminum, plastics... • heat can destroy magnetism

9. The earth is a big magnet • The earth’s north geographic pole is the south pole of a big magnet. • A compass needle is attracted to the earth’s north geographic pole • The earth’s magnetism is due to currents flowing in its molten core (not entirely understood!) the magnetic north pole is inclined about 14° from the geographic north pole, or by about 600 miles.

10. Report: Earth's magnetic field fading Slight chance of flipping magnetic poles CNN Report Dec. 12, 2003 North time South The strength of the earth’s magnetic field had Decreased10% over the last 150 years. At this rate, the field will disappear altogether in 1,500 to 2,000 years.

11. Sun – Earth Connection:space weather earth solar eruption SUN solar eruption Northern Lights

12. Solar eruptions

13. Particles stay on the magneticfield lines electron that originated on the sun  part of the solar wind. The earth’s magnetic field traps charged particles. During periods of intense solar activity, satellites can be at risk.

14. Van Allen Radiation Belts

15. Magnetic forces • magnetic fields can cause charges to turn around.

16. Magnetic deflection of electrons in a TV tube.

17. S S N N Magnetic forces on wires Wire pushed OUT Wire pulled IN

18. Forces on parallel wires Opposites repel Likes attract

19. Applications Magnets are not just for holding things on the refrigerator!

20. The electric motor When a current is present in a coil, it experiences a torque and rotates.

21. Magnets make speakers work The force between the permanent magnet and the voice coil moves the speaker cone

22. MAGLEVTrains

23. Magnetic tape recording